Non-chemical restoration techniques in California sage scrub: Testing variations of agricultural solarization adapted for use in wildlands

Background/Question/Methods Coastal sage scrub (CSS) is one of the most highly disturbed vegetation communities in California. Many former CSS sites are heavily invaded by exotic annual grasses and forbs that form dense stands, prohibiting establishment of native shrubs and forbs. Exotic propagules greatly outnumber native seeds in the seedbank. Some managers may not use herbicides for CSS restoration and need alternate methods for controlling invasives. Prior studies have shown success using the agricultural technique of solarization which uses clear plastic over moist soil in the summer. This heats the top layer of soil, killing seed. Another study reduced invasive species using black plastic during the winter growing season in the Mediterranean climate of California. Our goal was to compare the success of black or clear plastic, season of application and level of soil disturbance in reducing exotic seeds in the seedbank. All treatments were done without the addition of supplemental water, as the ability to irrigate in wildland scenarios is highly unlikely. The design used was a randomized complete block with 3 plastic treatments (black , clear and no plastic), 3 soil disturbance levels (tilling, scraping and mowing/ no disturbance) and 2 seasons of application (winter and summer). Results/Conclusions Preliminary results show that clear plastic placed in the summer controlled the most species overall. This occurred even though the soil was dry at the time of plastic placement due to drought conditions. This was surprising as wet soil is considered crucial to for conducting the heat needed to kill seed. Erodium cicutarium, one of the targeted exotic forbs, did emerge in clear plastic summer treated plots, however cover was less than 50%, which leaves bareground for seeded natives to establish. Black plastic in the winter reduced exotic grasses, but had greater survival of forbs both native and exotic. The results will help inform restoration practitioners who are limited by water and timing in controlling exotic seedbanks.